Knife sharpener



FIG. 1 is a perspective view of a knife sharpener showing my new design;

FIG. 2 is a top plan view thereof, the bottom surface of the sharpening head being substantially flat and plain;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is a front elevational view thereof. 

The ornamental design for a knife sharpener, as shown and described. 